Embodiments of the present invention relate to thin film batteries, such as solid state lithium batteries, and their fabrication and packaging.
Thin film batteries are used in applications that require a small battery with a high energy density such as, for example, portable electronics, medical devices and space systems. A solid state battery is a thin film battery composed of solid materials and which is generally absent liquid electrolytes. A thin film, sold state battery comprises a support having one or more battery cells, each battery cell comprising a set of battery component films which cooperate to store electrical charge and generate a voltage. The battery component films include an electrolyte sandwiched between electrodes, and can include metal-containing films composed of elemental metal, metal oxide or other metal-containing compounds. For example, elemental metal films can be used as current collectors to receive or provide electrons, such as for example, cathode and anode current collectors. The cathode or electrolyte can be metal oxide or metal-containing compounds. Thin film batteries have thicknesses smaller than the thickness of conventional batteries with battery component films thicknesses of less than 1000 microns, or even less than 100 microns. The battery component films are often formed by processes such as physical and chemical vapor deposition (PVD or CVD), oxidation, nitridation, and electroplating processes. These batteries can either be used individually or stacked together to provide more power or energy.
In thin film battery fabrication processes, the battery component films can be exposed to heat during processing or heat treated to anneal, re-crystallize, or reduce lattice defects. For example, elemental metal films are heat treated to reduce lattice defects and provide better conductivity. Metal oxide films are sometimes heated in air to anneal and/or obtain better crystalline properties. As an example, a cathode comprising a metal oxide electrode, such as a lithium cobalt oxide, provides better electrical properties when annealed in oxygen-containing environment at temperatures ranging from 300 to 700° C. It is believed that the electrical characteristics of the annealed cathode are related to its oxygen content and crystallographic structure, and when annealed, the cathode can allow the battery capacity to reach as high as its theoretical value for a given thickness and area. Still further, the lithium oxide film can be deposited to form a thicker cathode by depositing an amorphous or microcrystalline film, or a stack of sequentially deposited thin films, and thereafter, crystallizing the amorphous film or stack of films by heating. Increasing the thickness of a lithium oxide cathode film increases the energy density of the battery by as the thicker cathode provides greater charge retention and faster charging and discharging, as for example, taught in commonly assigned U.S. Pat. No. 7,186,479, entitled “THIN FILM BATTERY AND METHOD OF MANUFACTURE” to Krasnov et al., which is incorporated by reference herein in its entirety.
However, such heating and heat treatment processes often occur after deposition of underlying battery component films, and as such, can result in thermal degradation or oxidation of underlying layers. For example, heat treatment of an overlying cathode of metal oxide can cause oxidation of any underlying metal layers. As an example, good electrical conductors such as aluminum and copper, partially oxidize when annealed in environments having low partial pressures of oxygen. Further, aluminum oxide, when formed, is a dielectric having a high electrical resistivity of 1×1014 square-cm, which renders even a thin layer of aluminum oxide undesirable for a battery cell. Similarly copper is also prone to oxidation at elevated temperatures and in oxygen containing environments.
Nonreactive noble metals, such as platinum or gold, have also been used in battery cells to prevent or reduce such thermal or oxidation degradation. For example, lithium batteries often use a cathode current collector composed of platinum underlying a metal oxide cathode which is heated in an oxygen-containing environment to anneal and/or crystallize the cathode material, as for example, described in commonly assigned U.S. Pat. No. 7,862,927, entitled “THIN FILM BATTERY AND MANUFACTURING METHOD” to Krasnov et al., which is incorporated by reference herein and in its entirety. Platinum avoids oxidation and remains in its electrically conductive elemental metal form without oxidizing even after being heated in an oxidizing environment.
However, cathode current collectors composed of noble materials, such as platinum or gold, can be costly and substantially increase the price of the battery. Still further, platinum can exhibit poor adhesion to certain battery supports. Also, the difference in thermal expansion coefficients between platinum and a battery support material can result in delamination of the deposited platinum film when heated. Thus an adhesive layer is often deposited on the battery support prior to deposition of the platinum layer to increase adhesion and reduce peeling-off. However, the additional deposition step required for forming the adhesion layer adds to fabrication costs and complexity.
For reasons including these and other deficiencies, and despite the development of various battery structures, and deposition and heat treatment processes for solid-state, thin film batteries, further improvements in such batteries and fabrication steps are continuously being sought.